1. Field of the Invention
The present invention relates to focused ion beam (FIB) methods and systems useful, for example, in failure analysis of Very Large Scale Integrated (VLSI) circuit devices. In particular, the present invention relates to methods and systems in which a FIB is employed in locating, milling, and/or depositing conductors within an integrated circuit.
2. The Prior Art
Time-to-market often determines the economic success or failure of new IC devices in the semiconductor industry. Reducing the debug and verification time for new IC designs provides an important competitive advantage. Systems based on Focused Ion Beam (FIB) technology and electron-beam (e-beam) technology are increasingly important in debug and verification as IC design rules shrink below 1 .mu.m.
Systems based on FIB technology are gaining widespread acceptance in the semiconductor industry in a broad range of applications including IC repair, failure analysis and process monitoring. See, for example, K. NIKAWA, Applications of Focused Ion Beam Technique to Failure Analysis of VLSIs: a Review, Paper presented at the SECOND JAPAN-US SEMINAR ON FOCUSED ION BEAM APPLICATIONS, December 1990. A number of general purpose commercial FIB systems are available, but none so far is believed to have an architecture tailored specifically to the needs of the semiconductor industry.
FIB systems are conventionally used to perform three major functions: (1) etching/milling of structure, such as for cutting metal lines and drilling holes, (2) depositing material, such as for forming metal connectors and pads, and (3) scanning ion microscope (SIM) observation. These functions may be employed to modify the IC for failure analysis. For example, cutting and connecting metal lines aids in confirmation of an suspected failure mechanism or failure location, and milling holes in an insulation layer allows a "buried" conductor to be exposed or connected to a pad for improved e-beam or mechanical probing.
However, FIB systems currently in use have significant limitations. In semiconductor fabrication processes where inter-layer dielectric material is used to planarize the IC's surface, conductors buried underneath the surface are often not visible in a conventional SIM image. Milling through the dielectric material to expose a buried conductor poses the difficulty of precisely positioning the FIB over the conductor without being able to observe the conductor.
The FIB IC repair success rate is well known to be poor (typically &lt;50%) particularly when IC modifications are complex or extensive. It is often necessary to modify three devices to obtain one device which works; a small increase in the success rate of modification can thus dramatically increase the time required to obtain a single functioning device. The milling process is sufficiently imprecise that it is possible to mill too deeply and cut through the conductor or mill too little and fail to expose the conductor. The result can be unintended open circuit within the modified IC. Unreliable end-point detection, i.e., unreliable determination of when to terminate the milling process, particularly on lower level conductors, is frequently cited as the key issue in failed repairs.
Repair of an IC may involve making multiple modifications, reducing the odds of success. Verifying that a repair has been correctly made involves removing the IC from the FIB system and, typically, placing it in an e-beam system where it may be stimulated and probed. Overall, the repair and verification process can be time-consuming and yield uncertain results.
In recent years, voltage-contrast electron-beam (e-beam) probing has gained broad acceptance in the semiconductor industry for applications including IC design debug and verification, failure analysis and characterization. See, for example, N. RICHARDSON, E-Beam Probing for VLSI Circuit Debug, VLSI SYSTEMS DESIGN, August 1987. E-beam probing systems and techniques allow signals on internal nodes of an operating IC to be probed, using the principle of voltage contrast in a scanning electron microscope (SEM). A focused beam of primary electrons is directed toward a conductor within a circuit specimen as signals are applied to the specimen. Detected secondary electrons are indicative of the surface electrical potential on conductors within the specimen. See, for example, E. Menzel & E. Kubalek, Fundamentals of Electron Beam Testing of Integrated Circuits, 5 SCANNING 103-122 (1983), and E. Plies & J. Otto, Voltage Measurement Inside Integrated Circuit Using Mechanical and Electron Probes, IV SCANNING ELECTRON MICROSCOPY 1491-1500 (1985).
Commercial introduction by Schlumberger in 1987 of the "IDS 5000.TM." workstation-based, electron-beam test probe system greatly simplified E-beam probing of circuit chips and increased the efficiency of circuit debug. Among other features, the IDS 5000 system offers tools for "navigation" of an IC so that the e-beam can be rapidly and precisely placed to probe a location of interest while the IC is being electrically stimulated. See S. CONCINA et al., Software Integration in a Workstation Based E-Beam Tester, INTERNATIONAL TEST CONFERENCE PROCEEDINGS PAPER 17.6 (1986); S. CONCINA et al., Workstation-Driven E-Beam Prober, INTERNATIONAL TEST CONFERENCE PROCEEDINGS Paper 23.1 (1987); J. McLeod, A New Tool Dramatically Cuts VLSI Debugging Time, ELECTRONICS Apr. 30, 1987); N. RICHARDSON, E-Beam Probing for VLSI Circuit Debug, VLSI SYSTEMS DESIGN (August, 1987); S. Concina & N. Richardson IDS 5000: an Integrated Diagnosis System for VLSI, 7 MICROELECTRONIC ENGINEERING (1987); C. TALBOT, VLSI Inspection Using Electron-Beam Imaging Systems, PRODUKTRONIKA 87, SENSORIK (1987); and see U.S. Pat. Nos. 4,706,019 and 4,721,909 to N. Richardson, which are incorporated herein by reference.
To summarize, an e-beam system such as the IDS 5000 system aids analysis of the operation of an IC, but does not offer the ability to modify the IC. Existing FIB systems offer the means to modify an IC, but pose difficulties in performing such steps as locating buried conductors, and determining when to terminate milling, and do not allow verification of the sufficiency of a repair.